This invention relates to fluorescent lamps of the type comprising an elongated discharge vessel. The discharge vessel encloses a discharge path. The discharge vessel contains a gas fill which is excitable from a non-excited state into an excited state by a discharge arc in the operating state of the lamp.
The invention further relates to a method for starting a discharge arc through the elongated discharge path in the discharge vessel of such fluorescent lamp.
Low pressure discharge lamps are well known in the art. These lamps contain a gas fill which radiates UV light when excited by a discharge arc. The UV light is converted to visible light by a suitable light powder on the surfaces of the discharge vessel which is made by glass in most cases. The discharge arc is generated by a suitable voltage applied to a pair of electrodes at the two ends of the discharge path. The achieved light output is a function of the length of the discharge path, and it is sought to make the discharge path as long as possible.
However, a long discharge path requires a relatively high starting voltage between the electrodes of the lamp. In turn, the high voltage requires special electronic circuits because the high voltage must be applied only during the start-up phase of the discharge. As the discharge arc develops and the gas fill is excited, the overall impedance across the discharge arc drops, and a relatively low voltage level is sufficient to maintain the discharge process in the lamp.
Therefore, when the discharge across the discharge path has stabilized, the built-in electronics in the lamp detects the current level through the discharge path, and reduces the voltage applied to the electrodes. These electronics are not only expensive, but also bulky. The electronics system could be significantly simplified if the starting voltage across the electrodes of the discharge vessel could be lower.
A flat compact fluorescent lamp is disclosed in U.S. Pat. No. 5,767,618. This lamp contains a gas fill which is enclosed in a discharge vessel. A spiral-shaped discharge path is formed in the discharge vessel which latter is constituted by a bottom and top panel. The discharge path contains adjacent sections which are separated from each other by a convoluted wall. The wall is a part of the bottom panel, and an edge of the wall is in close proximity to the top panel, so a narrow gap exists between the wall and the top panel. It is recognized in U.S. Pat. No. 5,767,618 that arching may develop across the gap between the wall and the top panel. This arching is regarded as a negative effect. The U.S. Pat. No. 5,767,618 describes a lamp structure where the arching is suppressed. It is not recognized or implied that the cross-arching between the adjacent sections of the lamp could be put to use during the start-up phase of the lamp.
Therefore, there is a particular need for a method for starting the discharge arc in fluorescent lamps with a relatively reduced voltage, so that the electronics of the lamp could be made simpler and cheaper, or some parts of the electronics could be dispensed with completely. Also, there is a need for a fluorescent lamp which would require lower starting voltage, and which at the same time may be manufactured in a simple manner with existing technologies.
In an embodiment of the present invention, there is provided a fluorescent lamp comprising an elongated discharge vessel which encloses a discharge path. The discharge vessel contains a gas fill. The gas fill is excitable from a non-excited state into an excited state by a discharge arc in the operating state of the lamp. The discharge path has a first impedance in a non-excited state. The gas fill is of the type where the impedance of the gas is lower in the excited state than in the non-excited state. The discharge vessel has at least two sections located adjacent to each other, and electrode means for generating a discharge arc across the discharge path in the discharge vessel. There is bypass means for providing a bypass path for the gas discharge during a startup of the lamp between the two adjacent sections of the discharge vessel. The bypass path results in a short-cut across the impedance of at least a portion of the discharge path when the gas in said portion is in the non-excited state.
According to another embodiment of the invention, there is provided a method for starting a discharge arc through an elongated discharge path in a discharge vessel of a fluorescent lamp, where the discharge vessel contains a gas fill which is excitable from a non-excited state into an excited state by the discharge arc in the operating state of the lamp. The method is applicable with such gas fills where the impedance of the gas fill is lower in the excited state than in the non-excited state. The method comprises the following steps. A voltage is applied between two electrodes across the discharge path in the discharge vessel, where the gas fill in the discharge path between the electrodes has a first impedance in a non-excited state. A bypass path is provided between two ends of a bypassed part of the discharge path, thereby dividing at least a portion of the discharge path into a bypassed part and at least one remaining part. The combined impedance of the bypass path and the associated bypassed part is selected to be lower than the impedance of the associated bypassed part of the elongated discharge vessel, when the gas in at least a part of said bypassed part is in the non-excited state. Thereby a relatively increased voltage is provided across the remaining part of the discharge path. The gas fill in the remaining part of the discharge path is excited into the excited state with the help of the increased voltage across the remaining part. As a result, the impedance of the remaining part is lowered and the voltage across the bypassed part is relatively increased. Finally, the gas fill is excited in at least a part of the bypassed part by the relatively increased voltage across the bypassed part.
The suggested fluorescent lamp thus requires a lower starting voltage on the electrodes because the available starting voltage needs not generate the discharge arc across the full length of the discharge path. Instead, the discharge path is effectively divided into shorter sections which are started after each other. The shorter sections have a lower impedance, and may be excited by a lower starting voltage.
The method and the fluorescent lamp implementing the method ensures a gradual, softer startup of the discharge arc, while the lamp may be manufactured at a lower cost.